Allowing subsequent property declarations of compatible types

Search Terms

TS2717, subsequent property declarations

Suggestion

Let's assume following code:

Library typings:

interface Dictionary<T> {
   [key: string]: T;
}
interface Foo {
   methods: Dictionary<Function>;
}

User extension:

declare module "@awesome/library" {
     interface BagOfFunctions extends Dictionary<Function> {
         wellKnownFunction(foo: number): number;
     }
     
     interface Foo {
           methods: BagOfFunctions;
      }
}

Currently, it results in TS2717. After this change, such code will be allowed.

Use Cases

Use case: Hapi allow user to define global functions with shared cache (server.method).

Unfortunely, server.methods is currently defined as Dictionary<ServerMethod> and end users aren't able to get type safety when calling them.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Inizia con la diagnostica TS2717 e il comportamento di merging delle dichiarazioni mostrato negli esempi. Confronta il rifiuto attuale di Foo.methods con l’accettazione richiesta di BagOfFunctions, quindi definisci la copertura per le redeclarazioni compatibili e incompatibili. Il lavoro è completato quando l’estensione di esempio viene accettata senza indebolire gli errori relativi alle proprietà incompatibili.